I believe they are doing some search aggregation from multiple sources and from what they say they do not seem to be caching results much.
> You must use results you obtain through the Services only in Internet Search Experiences (as defined in the use and display requirements) and must not cache or copy results. (https://www.microsoft.com/en-us/bing/apis/legal)
It's really unfortunate Microsoft doesn't provide a free API that require users to display ads, or at the very least make their ad network accessible to all.